Fair Value (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 2,626.2	$ 1,956.4
Equity Securities	2,468.2	3,632.0
Other invested assets, current	23.8	17.2
Securities lending collateral	624.6	455.1
Total assets	23,323.7	24,001.9
Total liabilities	(17.8)	(21.5)
Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	631.0	1,300.3
Fixed Maturity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	599.9	790.6
Common Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	925.2	1,253.7
Private Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	312.1	287.4
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	17,569.1	17,938.1
Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	491.7	645.5
Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	109.9	90.1
Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,165.0	6,034.9
Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	7,782.5	7,485.8
Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,800.6	2,538.9
Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	73.3	78.7
Fixed Maturities [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,146.1	1,064.2
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	2,468.2	3,632.0
Equity Securities [Member] | Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	631.0	1,300.3
Equity Securities [Member] | Fixed Maturity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	599.9	790.6
Equity Securities [Member] | Common Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	925.2	1,253.7
Equity Securities [Member] | Private Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	312.1	287.4
Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	23.8	17.2
Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	11.8	3.1
Derivative liabilities	(17.8)	(21.5)
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,626.2	1,956.4
Securities lending collateral	473.9	214.1
Total assets	4,591.9	4,734.6
Total liabilities	0.0	0.0
Level I [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2.8	100.0
Level I [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level I [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level I [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level I [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2.8	24.8
Level I [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level I [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level I [Member] | Fixed Maturities [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	75.2
Level I [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	1,465.2	2,446.9
Level I [Member] | Equity Securities [Member] | Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	631.0	1,300.3
Level I [Member] | Equity Securities [Member] | Fixed Maturity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level I [Member] | Equity Securities [Member] | Common Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	834.2	1,146.6
Level I [Member] | Equity Securities [Member] | Private Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level I [Member] | Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	23.8	17.2
Level I [Member] | Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	0.0
Derivative liabilities	0.0	0.0
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0.0	0.0
Securities lending collateral	150.7	241.0
Total assets	18,085.2	18,729.8
Total liabilities	(17.8)	(21.5)
Level II [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	17,231.8	17,588.0
Level II [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	491.7	645.5
Level II [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	109.9	90.1
Level II [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,165.0	6,034.9
Level II [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	7,475.3	7,231.8
Level II [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,796.1	2,533.9
Level II [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	73.3	78.7
Level II [Member] | Fixed Maturities [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,120.5	973.1
Level II [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	690.9	897.7
Level II [Member] | Equity Securities [Member] | Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level II [Member] | Equity Securities [Member] | Fixed Maturity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	599.9	790.6
Level II [Member] | Equity Securities [Member] | Common Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	91.0	107.1
Level II [Member] | Equity Securities [Member] | Private Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level II [Member] | Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	0.0	0.0
Level II [Member] | Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	11.8	3.1
Derivative liabilities	(17.8)	(21.5)
Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0.0	0.0
Securities lending collateral	0.0	0.0
Total assets	646.6	537.5
Total liabilities	0.0	0.0
Level III [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	334.5	250.1
Level III [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level III [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level III [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level III [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	304.4	229.2
Level III [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4.5	5.0
Level III [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Level III [Member] | Fixed Maturities [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	25.6	15.9
Level III [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	312.1	287.4
Level III [Member] | Equity Securities [Member] | Exchange Traded Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level III [Member] | Equity Securities [Member] | Fixed Maturity Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level III [Member] | Equity Securities [Member] | Common Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	0.0	0.0
Level III [Member] | Equity Securities [Member] | Private Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities	312.1	287.4
Level III [Member] | Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	0.0	0.0
Level III [Member] | Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0.0	0.0
Derivative liabilities	$ 0.0	$ 0.0